Application of nitride semiconductors having a high saturation electron velocity and a wide band gap to high-voltage, high-power semiconductor devices is being considered. For example, GaN, which is a nitride semiconductor, has a band gap of 3.4 eV that is greater than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs, and has a high breakdown field strength. For this reason, GaN is a very promising material for a high-voltage-operation, high-power semiconductor device for a power supply.
Many reports have been made on field effect transistors, particularly, high electron mobility transistors (HEMT), which are examples of semiconductor devices using nitride semiconductors.
For example, a GaN HEMT, which uses GaN as an electron transit layer and AlGaN (InAlGaN) as an electron supply layer, is getting attention (see, for example, Japanese Laid-Open Patent Publication No. 2007-200975 and Japanese Laid-Open Patent Publication No. 2005-302861).
In a HEMT, due to the balance between the amounts of polarization charge in the electron transit layer and the electron supply layer, a high-concentration of two-dimensional electron gas (2DEG) is generated near the interface between the electron transit layer and the electron supply layer. The electron mobility of the 2DEG influences the performance of a transistor. As the electron mobility of the 2DEG increases, the current driving force of a transistor increases. To improve the electron mobility of the 2DEG, a spacer layer may be provided between the electron supply layer and the electron transit layer. The spacer layer has a low electron affinity and functions as a barrier against the 2DEG. Accordingly, providing the spacer layer between the electron supply layer and the electron transit layer prevents penetration of the wave function of the 2DEG into the electron supply layer. This in turn reduces the access resistance (channel resistance) and improves the electron mobility.